# analog measuring with low voltage arduino supply

Hello,

I need to measure voltage between 0-5V
the problem is sometimes, 5v supply can go down to 4V, the arduino works, but measurment is wrong because voltage reference is not 5V anymore
Can I supply 5V on Aref, have 4V on "5V power input" pin, and measure correctly 4.5V on a analog pin?

thank you

altzu:
Can I supply 5V on Aref, have 4V on “5V power input” pin, and measure correctly 4.5V on a analog pin?

No.

The answer is to lower the input voltage with a voltage divider to <= 1volt.
Then switch the A/D reference internally (with code) to 1.1voltAref in setup().
That makes things also independent of supply voltage variations.
Leo…

OK, thank you,

pedemuriff:
OK, thank you,

Input voltages between ~1.1volt and 5volt VCC will return the max A/D value of 1023.
Leo..

If the vRef is set to 1.1V then to get a full range of measure from 0 to 5 V a resistor divider could be used.

voltage divider calculator

Just entering 5V for a source volt, 10K for R1 and 1.1V for the output V's will give a starting point.

Another benefit of the internal reference is the input is still protected to Vcc (5v), despite having a full scale voltage of ~1.1volts .

So if you arrange the input to your resistor divider to give you 1v with a 5v volt input , that would mean you could put 25v on the divider and not damage the Arduino ( as that would still only give 5v on the analog input)

Note that different devices have a large variation of the 1.1V reference, so you have to calibrate your
Arduino specifically, as the nominal 1.1V can be anything from 1.0V to 1.2V according to the datasheet.

[ This would be a big issue for mass-production ]

You might also want to consider external voltage references which are commonly available in 4.096V, 2.56V,
or simply using the 3.3V pin as a 3.3V reference?

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